The temperature conditioning demand on many refrigeration systems can vary substantially. This is especially true in residential and commercial applications where air cnditioners and heat pumps experience seasonal changes, changes in humidity, diurnal changes, and variations in occupancy. As a result, several methods have been developed which vary the capacity of the system to meet its demand. One method that emerges as being relatively efficient and versatile varies the capacity by modulating the speed of the system's refrigeration compressor.
Possibly the most common method of varying the speed of a compresor is by way of an inverter driven AC induction motor. The speed of the compressor is varied as the inverter varies the frequency of the current supplied to the compressor's motor. Although inverter driven motors have been in use for many years, several of their drawbacks have not yet been eliminated. Inverters require extensive circuitry that is relatively expensive, and in many cases, they product more line interference than other available drives. Moreover, many inverter driven induction motors are less efficient than variable speed DC motors.
The speed of a conventional DC motor is simply changed by varying the amplitude of its DC supply voltage. The motor includes brushes and a commutator which mechanically commutate the DC supply to become synchronized with the speed of the motor. Unfortunately, a problem arises from commutating the motor using mechanical means. Brushes wear out and are impossible to replace when installed within a hermetic shell of a refrigeration compressor. In addition, electrical arching between the commutator and the brushes generates electrical noise and creates chemical impurities in the refrigerant environment within the hermetic shell.
A permanent magnet brushless DC motor uses electronic commutation to overcome the problems associated with mechanically commutating a motor. Electronic commutation involves replacing the brushes and commutator with an electronic switching means that switches the voltage supplied to the motor leads in response to the rotor's position. The speed of the electronically commutated motor is controlled by varying the DC supply voltage, often with reference to a presumably predictable voltage/speed relationship. This relationship, however, can vary as it is affected by the load applied to the motor. Consequently, speed control schemes based on the presumed voltage/speed relationship are inadequate in systems subject to widely varying loads, such as air conditioners and heat pumps. Yet to directly sense the motor's load, which equals its speed times its torque, involves additional sensors and expense.